The function, reliability and performance of semiconductor devices depend on the use of semiconductor materials and surfaces which are clean and uniform. Billions of dollars and countless man-hours have been spent developing, characterizing, and optimizing systems and processes for fabricating and processing semiconductor materials. A primary goal of this activity has been the fabrication of materials and surfaces that are extremely clean and that have properties that are uniform, or vary uniformly, across the entire wafer. In order to characterize and optimize these processes it is necessary to be able to inspect and measure surface or bulk cleanliness and uniformity. For real-time process control, it is necessary to be able to make many measurements across a surface at high speed, and to do so in a manner that does not damage or contaminate the semiconductor surface. It is also highly desirable to be able to discriminate and classify different types of non-uniformities or contaminants. Classification is extremely important. Information on the nature of a non-uniformity can be used to determine if the non-uniformity might impact device performance or manufacturing yield. Classification information can also be used to identify the source of the non-uniformity.
One method of inspecting and measuring surfaces utilizes a non-vibrating contact potential difference sensor. The non-vibrating contact potential difference sensor consists of a conductive probe that is positioned close to a surface, and is electrically connected to the surface. The probe and the surface form a capacitor. A potential difference is formed between the probe tip and the surface due to the difference in work functions or surface potentials of the two materials. The probe tip is translated parallel to the surface, or the surface is translated beneath the probe. Changes in the work function or surface potential at different points on the surface result in changes in potential between the surface and the probe tip. Also, changes in the distance between the probe tip and the wafer surface results in changes in the capacitance. Changes in either the potential or capacitance between the probe tip and the wafer surface causes a current to flow into the probe tip. This current is amplified, converted to a voltage, and sampled to form a continuous stream of data which represents changes across the surface. The non-vibrating contact potential difference sensor can provide a continuous stream of data at rates greater than 100,000 samples per second. High data acquisition rates permit high-resolution images of whole semiconductor wafers to be acquired in only a few minutes.
The non-vibrating contact potential difference sensor produces a signal that is a combination of two characteristics of the measured surface-changes in surface potential and changes in surface height. The charge on the probe tip is determined as follows:Q=CV   (1)
Where Q is the charge on the probe tip, C is the capacitance between the probe tip and the measured surface, and V is the potential difference between the probe tip and the surface.
The current, i, into the probe tip is the derivative of the charge on the probe tip and is given by the following formula:
                    i        =                                            ⅆ              Q                                      ⅆ              t                                =                                    C              ⁢                                                ⅆ                  V                                                  ⅆ                  t                                                      +                          V              ⁢                                                ⅆ                  C                                                  ⅆ                  t                                                                                        (        2        )            
The current, i, is the sum of two terms: the dV/dt term and the dC/dt term. The dV/dt term represents changes in the voltage between the probe tip and the wafer surface, and the dC/dt term represents changes in the capacitance between the probe tip and the wafer surface. The potential of the probe tip is fixed during the scanning operation, so changes in the dV/dt term arise from changes in the potential of the measured surface. Changes in capacitance result from changes in the distance between the probe tip and the wafer surface, which most often result from changes in the height of the wafer surface. This formula illustrates how the current into the sensor is a combination of changes in the potential and height of the measured surface.
Changes in surface potential can result from a variety of changes in surface and subsurface conditions. These include; but are not limited to; contamination by metals, contamination by non-metals or organics, changes in surface chemistry, changes in the number or type of molecules from the environment that adsorb on the surface, changes in the chemical termination of the surface, charging on the surface, charging in a dielectric deposited on the surface, changes in the atomic roughness of the surface, changes in surface film stress, changes in subsurface doping density or implant dose, changes in subsurface doping or implant depth, changes in potential at subsurface interfaces, changes in subsurface electrical conductivity, changes in crystalline structure or damage, changes in surface illumination, or some combination of these factors.
The non-vibrating contact potential difference sensor system can detect a wide range of surface and subsurface non-uniformities. However, it would be desirable to enhance the capabilities of the non-vibrating contact potential difference sensor inspection system to enable the discrimination and classification of different types of non-uniformities. For example, it would be desirable to discriminate between surface potential non-uniformities resulting from metallic contamination and surface potential non-uniformities resulting from organic contamination. As a second example, it would be desirable to discriminate between surface chemical contamination and variations in the height of the wafer surface.